This section provides background information which is not necessarily prior art to the inventive concepts associated with the present disclosure.
Lift gates provide convenient access to the cargo areas of hatchbacks, wagons, and other utility vehicles. Typically, the lift gate is hand operated and requires manual effort to be moved between open and closed positions. Depending on the size and weight of the lift gate, this effort can be difficult for some users. Additionally, manually opening or closing a lift gate can be inconvenient, particularly when the user's hands are occupied.
To reduce the effort and inconvenience of manually opening a lift gate, gas struts can be mounted between the vehicle body and the lift gate to reduce the force required to raise the lift gate. However, gas struts also hinder efforts to subsequently close the lift gate since the gas struts re-pressurize upon closing, thereby increasing the effort required. Additionally, the efficacy of gas struts varies according to the ambient temperature. Furthermore, the use of gas struts still requires that the lift gate is manually opened and closed.
As an alternative to manually-operated lift gates, some vehicles are equipped with a power lift gate system which typically include one or more power actuators configured to apply a force to the lift gate to control movement between the open and closed position. For example, U.S. Pat. No. 6,516,567 discloses a power actuator that works in tandem with a gas strut. The power actuator includes an electric motor mounted within the vehicle body that is coupled, via a flexible rotary cable, to an extensible strut that is pivotally mounted between the vehicle body and the lift gate. The electric motor can be controlled to raise and lower the lift gate without manual effort. A controller unit controls actuation of the electric motor and can be in communication with a remote key fob button or a button located in the passenger compartment or on the lift gate itself. However, this type of power actuator is not without its disadvantages. For example, the vehicle body must be specifically designed to provide a space to house the electric motor. Due to the limited space available, the motor is typically small and underpowered, thereby still requiring the assistance of a gas strut. Additionally, because the power actuator is designed to work in tandem with a gas strut, the gas strut can still vary in efficacy due to temperature. As such, the electric motor must be balanced to provide the correct amount of power with varying degrees of mechanical assistance from the gas strut.
U.S. Publication No. US2004/0084265 provides various examples of electromechanical power actuators for power lift gate systems. These electromechanical power actuators include an electric motor and a first reduction gearset coupled via a flexible rotary cable to a second reduction gearset which, in turn, is coupled via a slip clutch to a rotatable piston rod. Rotation of the piston rod causes a spindle drive mechanism to translate an extensible strut that is adapted to be pivotally mounted between the vehicle body and the lift gate. The slip clutch functions to permit the piston rod to rotate relative to the second gearset when a torque exceeding its preload is exerted on the lift gate so as to accommodate manual operation of the lift gate without damaging the electromechanical power actuator. A helical compression spring is installed in the power actuator to provide a counter balancing force against the weight of the lift gate.
U.S. Publication No. US2012/0000304 discloses several embodiments of power-operated drive mechanisms for moving trunk lids and lift gates between open and closed positions. The power-operated drive mechanisms have an offset configuration employing an electric motor-driven worm gearset to rotate an externally-threaded jackscrew for translating an extensible strut. A slip clutch is shown to be disposed between an output gear of the worm gearset and the rotatable jackscrew. In addition, a coupler unit is provided between the motor output shaft and the worm of the worm gearset. The coupler unit includes a first coupler member fixed for rotation with the worm shaft, a second coupler member fixed for rotation with the motor output shaft, and a resilient spider member interdigitated between fingers extending from the first and second coupler members. The coupler unit provides axial and circumferential isolation between the first and second coupler members and functions to absorb transient or torsional shock loads between the motor shaft and the worm shaft. While use of such a coupler unit provides enhanced damping characteristics and accommodates mis-alignments, the addition of such a coupler unit to the power-operated drive mechanism increases packaging requirements and assembly complexity.
Many such electromechanical power actuators require significant friction to provide a “stop and hold” lift gate function throughout the entire range of pivotal lift gate travel, regardless of environmental conditions and the vehicle grade. Typically, this friction is associated with the spindle-type drive mechanism, and not in the counterbalance mechanism, because the power lift gate system is either a dual drive arrangement or the friction limit on the counterbalance mechanism has already been reached. Specifically, back-drive friction can be added to the spindle-type drive mechanism by increasing the gear ratio associated with the reduction gearbox or by reducing the lead of the rotary power screw associated with the spindle-type drive mechanism. However, increasing these ratios also increases the motor size and speed requirements which can ultimately result in undesirable noise and back-EMF being generated by the motor during a manual closing of the lift gate.
While electromechanical struts currently used in powered lift gate closure systems provide enhanced convenience over non-powered manual lift gate closure systems, a need exists to continue development of improved power actuators which obviate or mitigate one or more of the shortcomings associated with prior art power actuators.